You may have seen TV shows in which a hospital worker or paramedic "shocks" an unconscious person out of cardiac arrest with a pair of electrified paddles. An implantable cardioverter-defibrillator (ICD) does the same thing, only internally and automatically when it detects that your heartbeat is abnormal.

An ICD is surgically placed under your skin, usually below your left collarbone. One or more flexible, insulated wires (leads) run from the ICD through your veins to your heart.

Because the ICD constantly monitors for abnormal heart rhythms and instantly attempts to correct them, it helps treat cardiac arrest, even if you're hours away from the nearest hospital.

How an ICD works

When you experience a rapid heartbeat, the wires from your heart to the device transmit signals to the ICD to send electrical pulses to regulate your heartbeat. Depending on the problem with your heartbeat, your ICD could be programmed for these therapies:

Low-energy pacing therapy. You may feel either nothing or a painless fluttering in your chest when your ICD responds to mild disruptions in your heartbeat.

Cardioversion therapy. A higher energy shock is delivered to deal with a more serious heart rhythm problem. You might feel as if you're being thumped in the chest.

Defibrillation therapy. This is the strongest form of electrical therapy used to restore a normal heartbeat. During this therapy you may feel as if you're being kicked in the chest. It may knock you off your feet. The pain from this therapy typically lasts only a second. There should be no discomfort after the shock is over.

Usually, only one shock is needed to restore a normal heartbeat. Sometimes, however, you may have two or more such shocks during a 24-hour period. Frequent shocks in a short time period are known as ICD storms. If you experience ICD storms, you should seek emergency care to see if your ICD is working properly or if you have a problem that's making your heart beat more abnormally. If necessary, the ICD can be adjusted to reduce the number and frequency of shocks. You may need additional medications to make your heart beat regularly and decrease the chance of an ICD storm.

Who needs an ICD?

You're a prime candidate for an ICD if you've had sustained ventricular tachycardia, survived a cardiac arrest or have fainted from a ventricular arrhythmia. You may also benefit from an ICD if you have:

A history of coronary artery disease and heart attack that has weakened your heart.

An inherited heart defect that makes your heart beat abnormally. These include long QT syndrome, which can cause ventricular fibrillation and death even in young people with no signs or symptoms of heart problems. Having other rare conditions such as Brugada syndrome and arrhythmogenic right ventricular dysplasia also may mean you need an ICD.

Banning AS, et al. Driving and arrhythmia: A review of scientific basis for international guidelines. European Heart Journal. 2013;34:236.

Epstein AE, et al. Addendum to ''Personal and public safety issues related to arrhythmias that may affect consciousness: Implications for regulation and physician recommendations — A medical/scientific statement from the American Heart Association and the North American Society of Pacing and Electrophysiology" public safety issues in patients with implantable defibrillators. Circulation. 2007;115:1170.

What is an implantable cardioverter-defibrillator (ICD)? American Heart Association. http://www.heart.org/HEARTORG/Conditions/Arrhythmia/PreventionTreatmentofArrhythmia/Implantable-Cardioverter-Defibrillator-ICD_UCM_448478_Article.jsp. Accessed July 10, 2015.

Ganz LI. General principles of the implantable cardioverter-defibrillator. http://www.uptodate.com/home. Accessed July 10, 2015.

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